Mac only: Adium, the cross-platform IM and communication client that's basically a Mac-styled cousin to Lifehacker favourite Pidgin, is out in official 1.4 form. Along with lots of fixes and upgrades from the long-running betas, there's Twitter and IRC support added here.
Adium does something interesting with Twitter in this release, integrating its message flow into IM-style protocol. Direct messages from contacts arrive as if they were IMs; removing a buddy from your Twitter list actually unfollows them from your account. You can change how often Adium looks for messages and how it handles your replies in the settings. Beyond that, Adium's changed a whole lot of features and fixed a heck of a lot of bugs, listed in the changelog linked here.
Adium 1.4 is a free download for Mac (1.5 and later) systems only.